Case Study Large onsite detention system for premium residential development

By Atlan Stormwater

Westwood Estate, an 11.5 ha site in Caddens, City of Penrith, is a masterplanned residential development with views of the scenic Blue Mountains. The site will include 119 premium lots that together form a sustainable community centre.

Stormwater management is a critical requirement for developments within the City of Penrith LGA. The council Stormwater Drainage Guidelines require OSD systems to ensure no increase in runoff from the site as a result of the development under all durations for all the storms up to and including the 1% AEP event.

Modelling

To determine the storage capacity of the OSD tanks, an analysis of both the pre-development and postdevelopment flows of the site was undertaken. The catchment areas for both the pre- and post-development scenarios were calculated, and the site’s hydrology was analysed using DRAINS modelling to determine the stormwater flow rates for both scenarios. The OSD tanks were sized and modelled in DRAINS to meet the council’s post-development flow requirements.

Atlan Stormwater is delivering the stormwater infrastructure for a sustainable residential development in the City of Penrith, NSW. Four large precast concrete onsite detention systems will be situated beneath bioretention systems, an approach that reduces both the horizontal footprint and the project timeline.

The design storm event was consistent with the requirements of Penrith City Council’s Design Guidelines for Engineering Works for Subdivisions and Developments. The conventional pit and pipe stormwater drainage system

(minor drainage) was designed to convey the 20% AEP storm event flows, with the road carriageway (major drainage) designed to convey flows exceeding the minor drainage event. The carriageway is designed with consideration for safe flow widths, ponding depths, and appropriate overflow locations.

Large on-site detention system needed

A precast OSD solution was the best option for this site due to the size of the required tanks. The eventual size of the OSD structures meant that a cast-in-situ solution would have been more difficult and time consuming to construct. A pre-cast OSD like the MegaVault tank is safer - less trades and less time with an open excavation, faster - days versus weeks, and cost effective - saving on capital costs and opportunity costs due to efficiency.

For large projects like this, cathedralstyle detention systems are preferable to a combination of smaller tanks, as the absence of internal walls saves concrete and space. The horizontal footprint is also minimised by the system’s highvolume vertical profile enabled by the three-metre internal height of each tank.

Four MegaVault tanks were installed within the site. Their locations were selected based on the existing catchment outflow locations for the site. The design aimed to align the OSD tanks and the discharge points for the tanks as close as possible to the existing discharge locations for the site, while not disrupting the current water flow for the area or creating nuisance flows for the adjoining properties.

Integrating bioretention and onsite detention

Bioretention systems will be constructed on top of the OSD tanks. This solution provides the required bioretention area and OSD volume while minimising the horizontal footprint. It also minimises the bioretention area depth, making maintenance easier.

Sub-soil drainage in the drainage layer will collect the water, which is then directed to stormwater pits in the bioretention area. These pits also collect water exceeding the extended detention depth of the bioretention basin. The stormwater pits connect to the pit riser at the top of the OSD tank, where water is stored and then discharged through an orifice outlet in the discharge control pit inside the OSD tank.

Looking to the future

The precast concrete systems, which are designed to last for 50 years, will future proof the estate, protecting the community by preventing localised flooding in storm events and severe downpours.